1. Field of the Invention
The present invention relates generally to medical systems and methods. More particularly, the present invention relates to the use of laser delivery systems for generating successive patterns of light energy for ablating corneal tissue.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ optical beam delivery systems for directing laser energy to a patient""s eye. The laser selectively ablate corneal tissue to reform the shape of the cornea and improve vision. Existing commercial systems employ pulsed lasers to ablate tissue from the eye. With these laser systems, each laser beam pulse ablates a crater in the tissue of the eye, and subsequent laser beam pulses ablate additional craters. A desired predetermined shape is sculpted in the corneal tissue by adjusting at least one of the position, size and shape of the craters made by the individual pulses.
The cornea includes an outer epithelial layer, a Bowman""s layer beneath the epithelial layer and a stromal layer beneath Bowman""s layer. At least a portion of the ablated predetermined shape is ablated in a layer beneath the epithelial layer. In order to achieve ablation of a layer beneath the epithelium, a layer of tissue is removed and a surface of the underlying tissue is exposed. This exposed surface of the cornea is ablated with the laser beam to sculpt a predetermined shape in the exposed surface. After the photorefractive keratectomy procedure, the epithelium rapidly regrows over the shaped area, producing a new anterior surface of the cornea. Alternatively, the epithelium is not removed but is partially severed and moved to the side for surgery and returned to its original position after the PRK.
The output beam from the lasers used in laser eye surgery systems is typically irregular and often requires treatment with special optics to create a more desirable beam. For example, the beams from the lasers are often spatially and temporally integrated in order to form a beam having uniform characteristics. In particular, the beams are integrated in order to display a flat or uniform intensity profile over a circular target region, often referred to as a xe2x80x9ctop hatxe2x80x9d profile. Alternatively, the laser beam may be cropped to select a portion of the beam having uniform characteristics, or the beam may be focused onto the eye to form a Gaussian energy profile distribution.
Once a desired beam shape is achieved, a laser beam may be used in different ways in order to effect corneal ablation. In a first type of system, the beam has a variable cross-sectional size. The maximum size generally corresponds to the total treatment area on the cornea. The beam size is manipulated using an iris or other exposure control mechanism, and the desired corneal reshaping can be achieved by properly controlling the exposure. Unfortunately, employing a laser beam having a size equal to the treatment area (typically on the order of 5.0 mm to 10.0 mm) requires the use of large, high pulse energy excimer lasers. Not only are such large lasers expensive, they also occupy a relatively large area, requiring significant space to house them. Another disadvantage of laser systems employing large uniform beams is that the shape ablated with a uniform beam is not necessarily uniform. For large diameter beams, work in connection with the present invention has suggested that the ablated shape may depend upon a variety of factors, including the hydration of the cornea. Unfortunately, hydration may vary across the surface of the cornea, and can be difficult to measure.
As an alternative to large variable size laser beam systems, laser scanning systems are also employed for corneal ablation. Scanning systems often employ a much smaller beam, minimizing energy required from the laser. The smaller lasers are also more economic and require less space. The use of a small beam width, however, complicates certain aspects of the treatment protocols. In particular, the laser pulses will partially overlap as the beam is scanned over the exposed surface of the cornea. This partial overlap of the beams causes the ablated craters to partially overlap and the ablated surface to become rough. Roughening of the ablated surface is undesirable because it can cause a cornea to scar and delay the recovery of visual acuity. Another disadvantage of this scanning approach has been the relatively small amount of tissue removed with each pulse of the laser beam. Because of the small amount of tissue removed, the laser must be used at very high pulse rates to keep the total treatment time within acceptable limits. These high pulse rates can cause tissue heating, resulting in scarring and loss of visual acuity.
Another approach has been to scan a variable size laser beam. This approach moves the laser beam over the treatment area while changing the size of the beam, and has been shown to be both effective and highly flexible. Unfortunately, this approach often involves fairly complex and expensive mechanical mechanisms and electrical circuitry. Additionally, overlap of the pulses often occurs during scanning, and this overlap causes the ablation to become rougher.
A disadvantage of the above scanning approaches is that a crater ablated by an individual pulse of the laser beam does not have a consistent curvature. This lack of consistent curvature in the ablated crater causes the exposed surface to become rougher as the tissue is sculpted to a desired shape. For example, scanning laser systems that have a laser beam energy profile with a uniform energy distribution will typically ablate individual craters having a steep wall and a flat central region. The peripheral region of the crater that includes the steep side wall has a very different curvature than the flat central region. Scanning laser systems with Gaussian or pseudo-gaussian laser beam energy profiles ablate craters having a cone-shaped edge with a rounded central region. The peripheral region of the crater (including the cone-shaped edge) again has a different curvature than the central region (including the rounded portion of the crater). The inconsistent curvature of a cornea ablated by these known scanning systems may limit the accuracy and benefit of resculpting procedures.
The use of large laser beams with a tailored energy density has also been suggested. First, it may be difficult to ablate complex shapes with this approach. Also, this approach requires the use of expensive lasers to produce large beams. As mentioned earlier, with large diameter ablations the ablated shape will depend upon the hydration of the cornea, and tissue hydration is difficult to measure. Consequently, this technique will produce variability in the ablated shape including central underablation that undesirably degrades visual acuity.
For the above reasons, it would be desirable to provide improved methods and systems for ablating corneal tissue. It would further be desirable to provide improved techniques for the scanning of light beams over corneal tissue in order to selectively ablate the tissue to treat vision disorders. In particular, it would be desirable to utilize small beam geometries with low pulse energy requirements while achieving a smooth ablation. Also, it would be desirable to more accurately ablate the surface to a desired shape with less dependence upon tissue hydration. Moreover, it would be desirable to simplify the control schemes and systems for scanning light beams for corneal treatment. At least some of these objectives will be met by the invention described hereinafter.
2. Description of the Background Art
Large beam variable width systems for performing photorefractive keratectomy (PRK) and phototherapeutic keratectomy are described in a number of patents including U.S. Pat. Nos. 4,973,330, 5,163,934, 4,732,148 and 4,729,372. A temporal and spatial beam integrator for a PRK/PTK laser system is described in U.S. Pat. No. 5,646,791.
Scanning systems for performing photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) are described in a number of patents, including U.S. Pat. No. 4,718,418 and 4,665,913. A laser surgical system employing a diffractive optical element adapted to an individual patient is described in U.S. Pat. No. 5,571,107.
Scanning variable width laser beam systems are described in issued U.S. Pat. No. 5,683,379 and a co-pending patent application entitled xe2x80x9cMethod and System for Laser Treatment of Refractive Errors Using Offset Imaging,xe2x80x9d U.S. patent application Ser. No. 08/058,599, filed on May 7, 1993, the full disclosures of which are herein incorporated by reference.
Laser ablation techniques using large width laser beams with a graded energy density are described in U.S. Pat. Nos. 5,219,343, 5,312,320, 5,207,668, 5,188,631 and 4,838,266, the full disclosures of which are herein incorporated by reference.
Use of a diffractive optical element is described in a co-pending applications entitled xe2x80x9cLaser Delivery System and Method with Diffractive Optic Beam Integration,xe2x80x9d U.S. patent application Ser. No. 09/015,841, filed on Jan. 29, 1998; xe2x80x9cMethod and System for Scanning Non-Overlapping Patterns of Laser Energy with Diffractive Optics,xe2x80x9d U.S. patent application Ser. No. 09/116,648, filed on Jul. 16, 1998, the full disclosures of which are herein incorporated by reference.
The full disclosures of each of the above-cited U.S. patents and applications are incorporated herein by reference.
The present invention provides improved methods, systems, and other apparatus for performing laser ablation. The present invention significantly improves the uniformity of treatment by scanning beams having energy distribution profile shapes that ablate craters with a consistent curvature. Preferably, the beams are scanned so as to cover the entire ablation zone with partially overlapping craters that have consistent curvature. The present invention provides a number of specific improvements over prior corneal ablation methods and systems. The technique provides for sculpting a surface with partially overlapping consistently curved craters. In some embodiments, the technique provides for sculpting a surface with partially overlapping uniformly curved craters. Typically, a laser sculpting to achieve a desired optical result will ablate the surface with a uniform or gradually varying change in curvature. For example, spherical corrections of near sightedness produce a uniform concave change in curvature, and the correction of hyperopia produces a uniform convex change in curvature. Advantageously, laser sculpting to correct an astigmatic curvature of the eye can exhibit a gradual change in curvature over the ablated surface. Similarly, other refractive errors and aberrations (such as mixed astigmatism, presbyopia and wavefront aberrations) may be treated with the technique without having to resort to incremental, stepped approximations of the desired smooth curvature.
In a first aspect, the invention provides methods for sculpting a region on a surface. The methods generally include directing pulsed beams toward the region and ablating craters with the beam pulses. The craters will often have a consistent curvature, the craters optionally being rounded and axissymmetric in shape. The beam is scanned over the region to effect a predetermined change in shape by partially overlapping the craters.
A dimension across the ablation craters is often about 5 to 80% of the dimension across the treatment region. In some embodiments, the curvature of the craters is substantially uniform and spherical, and the craters are of a substantially uniform size. The pulsed energy beam is preferably a laser beam. In some embodiments, the technique includes shaping the laser beam with a beam shaping element. In other embodiments, the technique includes diffracting the laser beam with a laser beam diffracting element.
In another aspect, the invention provides a laser system for sculpting an ablated region on a surface of a tissue to a predetermined shape. The laser system includes a pulsed laser for making a pulsed beam of an ablative laser energy, and a beam energy shaping element for changing a laser beam energy pattern of the pulsed beam to a shaped beam. The shaped beam includes a consistently curved laser beam energy pattern with a region of the consistently curved pattern above the threshold of ablation of the tissue. The system also includes a scanning element for moving the shaped beam over the region to sculpt the region with a plurality of partially overlapping pulses of the ablative energy.
The shaped beam may include a boundary enclosing the curved pattern and an intensity of the beam around the boundary may be a proportion of the threshold of ablation, the proportion being in a range of 100 to 150%. In some embodiments, the consistently curved laser beam pattern is a substantially spherical laser beam energy pattern.